The present invention relates to an X-ray tomographic imaging technique, and more particularly to an X-ray tomographic imaging system and method suitable for the inspection of minute defects in industrial parts, more especially, the inspection of defects in soldered portions between an electronic circuit module and a printed circuit board.
A typical example of the conventional industrial CT (computed tomography) apparatus is disclosed by Nakamura, "Industrial X-ray CT scanner and its applications", Keisoh, Vol. 27, No. 2, (1984), pp. 48-51. The disclosed apparatus includes an X-ray source which has a relatively larger size as compared with an object to be inspected and a multichannel array of X-ray detectors each of which contains a Xenon (Xe) gas filled therein and has its width of 1 to 2 mm. Usually, the detector array has 300 to 500 channels. By irradiating the object to be inspected with X-rays emitted in a sector shape from the X-ray source while rotating the object around its own axis, the intensities of X-rays transmitted through the object from the entire peripheral directions are detected by the X-ray detector array so that the detected data are used to reconstruct a cross-sectional image of the object to be inspected. However, the resolving power of this apparatus for the object to be inspected is on the order of 300 .mu.m.sup..phi.. Therefore, it was impossible to inspect relatively minute portions such as soldered portions at which an electronic circuit module and a printed circuit board are connected.
Another type of inspection apparatus using X-rays is disclosed by Tanimoto, "Recent non-destructive inspection techniques", in "Pipe Arrangement and Apparatus", Vol. 25, No. 3 (1985), pp. 54-60. In the disclosed X-ray TV (television) apparatus, internal defects of an object to be inspected non-destructively are inspected in superposition with images of those which are other than the object in question, by obtaining an internal image of the object from a mere X-ray transmission image or X-ray projection image which is acquired through the irradiation of the object with X-rays, and this is a problem in inspecting minute portions clearly. Further, when using the apparatus as X-ray CT scanner, there is a problem that the geometrical distortion of an X-ray inspection image produced by an X-ray fluorescence image intensifier and/or a variation in intensity of irradiation X-rays construct a factor of operation errors, thereby resulting in an obstacle in reconstructing the shape of a cross section of the object with a high precision.